Information about formation dip angles (“dip angles”) is a very important issue in oilfield exploration. In particular, dip angle information is used to determine the location of particular zones (e.g., shale zone, sand zone, etc.) within a formation. This information is subsequently used to determine whether a well is being drilled or can be drilled in an appropriate formation.
Dip angles are typically measured on a small scale (i.e., a few centimeters) or on a large scale (i.e., tens of meters). The measurement of dip angles on a small scale is typically conducted using well logging tools such as a Fullbore Formation MicroImager (FMI) tool, a Dipmeter tool, etc. The measurement of dip angles on a large scale is typically conducted using seismic equipment. Multiple well logs from one or multiple tools are typically required to determine the dip angles for a particular formation.
Dipmeters make high resolution micro-resistivity measurements around the borehole circumference, which are correlated to produce dip information. This is merged with tool orientation/navigation data to provide formation dips in the earth's frame of reference.
Dipmeters are commonly made in two sections. A lower caliper arm sub-section contains the mechanism for holding the dipmeter pads against the borehole wall, and the pads contain the micro-resistivity electrodes. An upper sub-section contains the magnetometers and level cells or accelerometers necessary to define the orientation of the tool in three dimensions. The two sections are joined in such a way as to prevent relative rotation.
A minimum of three circumferential measurements are needed to define a plane. Traditional slim dipmeters therefore have 3-arms 120° apart. Each caliper arm terminates in a pad from which a resistivity measurement is made. The pads themselves are made as short as possible to allow them to enter small cavities. Resistivities are measured with small laterolog-3 type arrays. The sense electrodes are typically located some distance above the caliper arms and sense the current returning to the body of the dipmeter. Pad traces are generally correlated automatically using an interval correlation technique. This can be augmented by machine-aided manual correlation.
A window of data on the reference pad (the “reference interval”) is correlated with corresponding intervals on the other pads, plus or minus an additional amount of data defined by a search angle. The reference interval is then moved by an amount known as a step.
The reference interval is typically determined by the information content of the data. For example, if the pad traces are poor as a result of intermittent contact in rugose conditions, the best results may come from a short interval rather than a long one. However, as a general rule, stratigraphic interpretations are more accurate using a short interval. The step and interval usually overlap by some fraction, commonly a half, e.g., for a 2 meter interval, use a 1 meter step. Some overlap is justified because correlatable features may fall at or near an interval boundary, and might not result in an identifiable peak on the correlogram if there is no overlap. Comparing plots obtained with and without overlap may be useful; however, there is little or no justification for more than two fold overlap.
The search angle is the angle above and below the interval on the reference pad which, when projected across the well, defines the trace lengths from the other pads which enter the correlation algorithm. Therefore, the search angle defines the maximum apparent dip that can be computed. Note that search angles are defined with respect to the borehole, so the borehole tilt is subtracted to find the maximum true dip angle that can be computed in a vertical well.
Once the dipmeter tool has traversed the depth of the well, or the area of interest within the well, a plurality of resistivity logs is produced. There is typically one dip angle calculated per step. By properly correlating the fluctuations of these resistivity logs, the positioning of a bedding plane relative to the tool position can be readily calculated. Then, by measuring the bearing of the tool relative to some azimuthal reference, such as magnetic north, and the inclination of the tool relative to the true vertical or gravitational axis, the position of a bedding plane relative to the north and true vertical axes can be determined. To obtain an accurate dip angle, performing accurate correlations of a number of signals is generally necessary.
In addition, some prior art modeling methods combine information from offset well logs, production data, geologic maps and cross-sections to generate an initial geometric framework. The geometric framework typically includes a basic model providing of the formation. The geometric framework is subsequently augmented with estimates obtained from seismic data, more detailed correlation studies, log plots from the pilot hole, etc. The additional data from the offset wells and the pilot holes provide information about the layer thickness and various layer properties. The layer boundaries are typically determined from the inflection points on the offset well logs and the average layer properties are extracted from the corresponding well log values. Further, the dip angles associated with the layer model are typically derived using a combination of geologic maps and a cross-section of the formation, oriented along the wellbore.